There is known an automatic lathe having a cylindrical workpiece holder to which a plurality of workpieces are fixed in respective positions and also having a plurality of machining heads corresponding to the positions where the workpieces are securely held on the cylindrical workpiece holder. In operation, the cylindrical workpiece holder rotates about its own axis to advance the workpieces respectively to next machining positions in which the machining heads machine the respective workpieces. The machining process for each workpiece is completed after it has been machined successively by the machining heads in the machining positions.
For example, an end surface of each workpiece is cut in a first machining position by a first machining head. After the end surface of the workpiece has been cut, the workpiece holder rotates to bring the workpiece to a second machining position. In the second machining position, a second machining head machines the workpiece roughly to a desired outside diameter. After the workpiece has been machined in the second machining position, the workpiece holder rotates to move the workpiece into a third machining position in which a third machining head machines the workpiece roughly to a desired inside diameter. In this manner, the workpiece holder rotates to bring the workpiece to a next machining position where the workpiece is machined in a next machining cycle. The above process is repeated until the workpiece is machined in a final machining position, whereupon the machining process for the workpiece is completed.
When a workpiece moves from one machining position to another, however, the relative position between the workpiece and each machining head varies, resulting in a reduction in the accuracy with which the workpiece is machined. The positioning accuracy of the workpiece with respect to each machining head may be increased by a mechanical means, but it would be highly difficult to position many workpieces relatively to each of the machining heads with a desired level of accuracy.